There are plenty of exercises to do with Dumbbells.Here are some Db Exercises you can do to get stronger:
Db Clean and Press
Db floor press
Db Skull crushers
Db High Pulls
Db Overhead Squats
Db Hammer curls
Db front squats
Db Stiff Legged deadlifts
Db crufix holds/side raises (w/pauses)
Db pull overs
Db concentration curls
* keep in mind you have the benefit of using one arm or leg at a time
Lots of strongman type trainings you can do as mentioned
You can get a keg and fill it with water and have lots of fun.
When the keg is too light fill it with sand and then add water.
Calisthenics as mentioned work well.